Measuring macro- micro-nutrients in soil with ICP-OES

Measuring macro- and micro-nutrients in soil with ICP-OES

5:57 AM, 26th September 2017
PerkinElmer’s Avio 200 ICP-OES.
PerkinElmer's Avio 200 ICP-OES.

By Ken Neubauer

Introduction

A key to producing abundant crops is the quality of the soil in which they are grown.  Soils lacking the proper nutrient content will yield fewer crops of lower nutritional value than crops grown in good soil. 

Poor soil can be enhanced by the application of fertilizer, but the nutrient content of the soil must be known in advance so that the proper type and amount of fertilizer can be added. 

When determining the elemental content of soils, both sample preparation and analysis must be considered. Sample preparation generally involves subjecting the soil samples to acid at elevated temperatures so that the elements of interest are extracted into the acid. This can be accomplished in two ways: open vessel or closed vessel.

Open vessel sample preparation involves heating the soil at sub-boiling temperatures for up to four hours and then separating the acid from the soil through filtration or centrifugation. The advantage of open vessel extraction is that a large number of samples can be prepared at the same time, but the disadvantages include having to separate the soil from the acid, the loss of volatile elements or elements in volatile forms, and possible incomplete extraction of the elements of interest.

Closed vessel microwave digestion is much faster, typically taking 50 minutes or less.  Because the vessels are sealed, elevated pressures are attained, resulting in temperatures significantly higher than the boiling point of the acids. The outcome is faster with more complete extractions.  If complete digestion is required, the choice of acids can be altered. Separating the acid from the remaining soil involves only decanting the acid, as the soil generally remains in the digestion tube. The only drawback of microwave digestion is that the number of samples which can be prepared at the same time is limited by the capacity of the microwave.

Typical nutrients in soil can be measured by either flame atomic absorption (FAA), inductively coupled plasma optical emission spectroscopy (ICP-OES), or inductively coupled plasma mass spectrometry (ICP-MS). Given that nutrients are present above trace levels, ICP-OES is usually the technique of choice for this analysis, providing the best compromise between analysis speed, sensitivity, and cost. This work focuses on the analysis of elemental nutrients in soils using closed-vessel microwave digestion using the Titan MPS™ Microwave and Avio® 200 ICP-OES.

Experimental

Samples and Sample Preparation

Soils were collected locally from residential yards and gardens, as well as commercial farms and pastures. The residential garden samples consisted of natural soils to which large quantities of “bagged” soils were added, as are commonly available at home and garden stores. All samples were taken from healthy, in-production plots, suggesting a high nutrient content. 

Two reference materials (Soil Solutions A and B, High Purity Standards, Charleston, SC, USA) were used to validate the accuracy of the methodology.

Sample preparation was done with closed-vessel microwave digestion using the Titan MPS Microwave Preparation System (PerkinElmer, Shelton, CT, USA). To each vessel, 1 gram of sample was added, along with pre-digestion analyte spikes as necessary for spike recovery studies, followed by 6 mL HCl (37 percent) and 3 mL HNO3 (70 percent). The vessels were allowed to stand for 10 minutes to allow any early reactions to occur before being sealed, placed in the microwave, and digested according to the program in Table 1. It should be noted that these parameters will not produce a complete digestion of the soil, but will extract the elements.  Complete digestion would be accomplished with the addition of hydrofluoric acid to break down the silicates.

After the digestion program completed, the vessels were removed from the microwave, uncapped, and the acid mixture decanted into 50 mL autosampler tubes. The vessels were triple-rinsed with deionized water to ensure complete transfer. The samples were then brought to final volume (50 mL) with deionized water. The sample tubes were allowed to sit for about 10 minutes to allow any particulates to settle.

Table 1:  Microwave Digestion Program

Step

Temp

(°C)

Pressure Limit (bar)

Ramp Time (min)

Hold Time (min)

Power

(%)

1

150

35

5

5

80

2

195

35

2

20

100

3

50

35

1

15

0


Instrumental Conditions

All analyses were performed with an Avio 200 ICP-OES (PerkinElmer, Shelton, CT, USA). The instrumental conditions are shown in Table 2, and the analytes, wavelengths, and plasma view modes appear in Table 3. The internal standard (yttrium, Y) was added on-line, with the total flow to the nebulizer being 1 mL/min. All standard sample introduction components and conditions were used.

Quantitative measurements were made against external calibration curves prepared in a 10% HCl/HNO3 mixture to approximate the acid content of the final samples. 

Table 2:  Instrumental Parameters

Parameter

Value

Nebulizer

Meinhard® Type K1, Glass

Spray Chamber

Baffled Cyclonic, Glass

Sample Uptake Rate

0.80 mL/min

Total Flow to Nebulizer

1.0 mL/min

RF Power

1500 W

Nebulizer Gas Flow

0.70 L/min

Auxiliary Gas Flow

0.2 L/min

Plasma Gas Flow

8 L/min

Table 3:  Element, Wavelength, and Plasma View Mode

Element

Wavelength (nm)

Plasma View

Al

308.215

Radial

Ba

233.527

Axial

Ca

317.993

Radial

Cu

327.393

Axial

Fe

238.204

Radial

K

766.490

Radial

Mg

285.213

Radial

Mn

257.610

Radial

Na

589.592

Radial

Ni

231.604

Axial

P

178.221

Axial

S

181.975

Axial

V

292.464

Axial

Zn

206.200

Axial

Y (int std)

371.029

Axial, Radial

Results and Discussion

The accuracy of the methodology was first assessed by analysis of the reference materials. Figure 1 shows that recoveries for all analytes in both reference materials were within 10 percent of the true values, demonstrating the accuracy of the methodology. However, since neither phosphorus (P) nor sulphur (S) were certified in these reference materials, the accuracy of these elements could not be ascertained. 

Next, the collected samples were digested and analyzed, with the results appearing in Figure 2. It is interesting to note both the similarities and differences in the nutrient content of these samples. While some elements showed consistency among the different samples (ie. Fe, K etc.), others showed more variation (ie. Ni, Zn etc.). It is also interesting to note the variations between the different sample types: backyard vs. garden vs. field.

To assess the accuracy of the sample preparation procedure, as well as validate the P and S measurements, analyte spikes were added to all samples prior to digestion, at the levels is shown in Table 4. The spikes were added directly to the digestion vessels after the addition of samples and acids. The recoveries, as shown in Figure 3, are all within 10 percent of their true values, demonstrating that no significant elemental loss or contamination occurred during the digestion process. In addition, the recoveries of P and S are within 10 percent of their spiked values, validating the accuracy of the methodology for these elements.

Figure 1:  Recoveries in two certified reference materials

Figure 2:  Results from analysis of collected soil samples

Figure 3:  Pre-digestion spike recoveries in collected soil samples

Table 4:  Pre-Digestion Spike Levels

Element

Spike Concentration (mg/L)

Ni, V, Zn

50

Cu

100

S

400

Ba, Mn, Na, P

500

Al, Ca, Fe, K, Mg

5000


Conclusions

This work, using the PerkinElmer Titan MPS Microwave and the Avio 200 ICP-OES, has demonstrated the ability to measure both macro- and micro-nutrients in soil. The microwave digestion provides fast, efficient analyte leaching by using elevated temperatures and pressures while preventing contamination and loss of volatile analytes. ICP-OES is an established analytical technique, well-suited to the challenges of nutrient analysis. By employing both axial and radial viewing, both macro- and micro- nutrients can easily be measured in soils. The combination of microwave digestion and ICP-OES has proven to be an ideal solution for measuring a wide linear range of elemental levels in soils.

Author: Ken Neubauer is Senior Application Scientist – Inorganic at PerkinElmer.

© Chemical Today Magazine


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